1. Field of the Invention
The present invention relates to a retardation optical element for use in a liquid crystal display or the like, particularly a retardation optical element having the function of reflecting ultraviolet light, capable of decreasing the amount of ultraviolet light that enters a liquid crystal cell, and to a liquid crystal display comprising such a retardation optical element.
2. Description of Related Art
FIG. 12 is an exploded, diagrammatic perspective view showing the structure of a conventional liquid crystal display.
As shown in FIG. 12, the conventional liquid crystal display 100 comprises a polarization layer 102A on the incident side, a polarization layer 102B on the emergent side, a liquid crystal cell 104, a back light unit 106, and a retardation layer 108.
Of these component parts, the polarization layers 102A and 102B are made so that they selectively transmit only linearly polarized light having a plane of vibration in a predetermined direction, and are arranged in the cross nicol disposition so that the direction of vibration of linearly polarized light which the polarization layer 102A transmits is perpendicular to that of vibration of linearly polarized light which the polarization layer 102B transmits. The liquid crystal cell 104 comprises a large number of cells corresponding to pixels and is placed between the polarization layers 102A and 102B. The retardation layer 108 is a birefringent layer useful, for example, for providing compensation for viewing angle dependency or the like, and is placed on one side, relative to the direction of thickness, of the liquid crystal cell 104. Besides, there also exists a liquid crystal display comprising retardation layers 108 that are placed on both sides, relative to the direction of thickness, of a liquid crystal cell 104.
The case where the liquid crystal cell 104 in the above-described liquid crystal display 100 is of VA (Vertical Alignment) mode, in which a nematic liquid crystal having negative dielectric anisotropy is sealed in the liquid crystal cell, is now taken as an example. Light emitted from the back light unit 106 passes through the polarization layer 102A on the incident side and becomes linearly polarized light. This linearly polarized light passes, without undergoing phase shift, through those cells in the liquid crystal cell 104 that are in the non-driven state, and is blocked by the polarization layer 102B on the emergent side. On the contrary, the linearly polarized light undergoes phase shift as it passes through those cells in the liquid crystal cell 104 that are in the driven state, and the light in an amount corresponding to the amount of this phase shift passes through and emerges from the polarization layer 102B on the emergent side. It is therefore possible to display the desired image on the emergent-side polarization layer 102B side by properly controlling the driving voltage that is applied to each cell in the liquid crystal cell 104. There exists not only a liquid crystal display 100 of the above-described type in which light is transmitted and blocked in the above-described manner, but also a liquid crystal display that is so constructed that light emerging from those cells in a liquid crystal cell 104 that are in the non-driven state passes through and emerges from a polarization layer 102B on the emergent side, and that light emerging from those cells that are in the driven state is blocked by the polarization layer 102B on the emergent side.
In general, a liquid crystal sealed in the liquid crystal cell 104 is apt to undergo deterioration by ultraviolet light, and its optical properties can change due to this deterioration. Specifically, for example, light emitted from the back light unit 106 using a conventional fluorescent lamp contains ultraviolet rays, and these ultraviolet rays enter the liquid crystal cell 104 via the polarization layer 102A on the incident side to deteriorate the liquid crystal in the liquid crystal cell 104. Moreover, sunlight and extraneous light such as light emitted from electric lamps (fluorescent lamps, etc.) also contain ultraviolet rays, and these ultraviolet rays also enter the liquid crystal cell 104 via the polarization layer 102B on the emergent side to deteriorate the liquid crystal contained in the liquid crystal cell 104. As the liquid crystal contained in the liquid crystal cell 104 deteriorates in this manner, the quality of the image displayed on the liquid crystal display 100 lowers.
Mercury in a fluorescent lamp emits rays of 185 nm, 254 nm, 305 nm and 365 nm, and it is known that, of these, a ray of 365 nm passes through the glass tube of a fluorescent lamp and is discharged to the outside. Further, sunlight contains rays that are classified, in the order of decreasing wavelength, into UVA (315 to 400 nm), UVB (280 to 315 nm) and UVC (100 to 280 nm). It has been considered that, of these rays, only UVA and UVB reach the surface of the earth and that UVC is absorbed by ozone and hardly reaches the surface of the earth. However, such a phenomenon that the ozone layer over Antarctica disappears is observed in recent years, and not only UVA and UVB but also UVC is now known to reach the surface of the earth.
Under these circumstances, there has been known a liquid crystal display in which an ultraviolet absorber is, in order to decrease the amount of ultraviolet light that enters a liquid crystal cell, incorporated in films such as polarization layers to be placed on both sides, relative to the direction of thickness, of the liquid crystal cell (see pages 1 to 4 of Japanese Laid-Open Patent Publication No. 80400/1997).
However, the liquid crystal display described in Japanese Laid-Open Patent Publication No. 80400/1997 has the following drawback: since an ultraviolet absorber is incorporated in films such as polarization layers, the film-forming process becomes complicated to increase the cost of production of the liquid crystal display.